Bleed-off (bypass) flow controls

Figure 10-40 shows a bleed-off or bypass flow-control circuit in the at-rest condition. This type of flow control circuit bleeds off excess fluid to tank. A bleed-off circuit works best in hydraulic circuits using fixed-volume pumps. And a bleed-off circuit only works with multiple actuators if they operate one at a time.

When oil passes to tank through a pressure-compensated flow control, cylinder movement will slow while system pressure only climbs high enough to move the load. (While this arrangement wastes energy, the amount is minimal.)

The cylinder in Figure 10-41 is extending at 3 gpm while 7 gpm passes to tank through the pressure-compensated needle valve. Because the resistance of the cylinder and load is only 100 psi, the energy needed is very low. The 3 gpm flowing to the cylinder generates no heat because it is doing useful work. The 7 gpm going to tank at a 100-psi pressure drop is the only wasted energy. If the cylinder were to contact a load that requires 300 psi then the whole system would climb to 300 psi. Energy losses would increase, but would still be much less than those in a meter-in or meter-out system.

Even with a pressure-compensated flow control valve, cylinder speed will change slightly as pressure increases. This is because pump flow decreases slightly when pressure increases. The flow control still passes 7 gpm. (All pump inefficiency losses reduce cylinder speed.) When the cylinder bottoms out, pressure increases until the relief valve opens. At this time all input energy generates heat. Note that this only happens when the cylinder must maintain force while stalled.

As with meter-in circuits, bleed-off circuits do not work with a running-away load. Figure 10-42 shows the cylinder extending rapidly and fully when the directional valve shifts. (When using a bleed-off circuit with a running-away load, use the counterbalance circuit shown in Figure 10-16. With a counterbalance valve creating resistance, oil entering the cylinder sets its speed as with any resistive load.)

Heat generation can be minimal if the cylinder force is low while extending and retracting. Figure 10-43 shows only 1038 BTU/hr loss -- even with 7 gpm being bled to tank. Normally, use a bleed-off circuit where only a small amount of fluid goes to tank to fine tune actuator speed.

A bleed-off circuit requires a pressure-compensated flow control to keep the cylinder from slowing as pressure increases. Without a pressure-compensated flow control, cylinder speed slows as load increases and speeds up when load decreases. With a bleed-off flow-control circuit, turn the flow control clockwise to increase speed and counter-clockwise to decrease speed.